Add host option with specs and update README

Added support for custom hosts in git URL [Issue #26](https://github.com/mattpolito/paratrooper/issues/26), for example `git@heroku.ACCOUNT_NAME:APP`.

This option allows one to use paratrooper with the heroku-accounts plugin or with any custom host/ssh pair.

I added specs and manually tested this by pushing to Heroku myself in a project using the heroku-accounts plugin. Works as expected!